1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, more specifically, to a light emitting device including a light emitting element, for example, an organic light emitting diode (OLED), formed on a plastic substrate. The present invention also relates to an OLED module in which an IC including a controller and the like is mounted onto an OLED panel. Throughout the specification, OLED panels and OLED modules are generically referred to as light emitting devices. The present invention further relates to an electrical appliance using the light emitting device.
2. Description of the Related Art
Recently, a technique for forming a TFT (thin film transistor) on a substrate has been remarkably developed, and continues to be further developed for its application to active matrix display devices. Particularly, a TFT using a polysilicon film can operate at a high speed because such a TFT has a higher field effect mobility than a TFT using a conventional amorphous silicon film. Therefore, the control of pixels, which has been conventionally performed by a driver circuit provided outside a substrate, can be now performed by a driver circuit provided on the same substrate on which the pixels are formed.
Such an active matrix display device includes various circuits or elements formed on the same substrate. Owing to this structure, the active matrix display device provides various advantages such as reduced manufacturing cost, reduced size of a display device, an increased yield, and an increased throughput.
Furthermore, an active matrix light emitting device including an OLED as a self-luminous element (hereinafter, simply referred to as a light emitting device) has been actively studied. The light emitting device is also called an organic EL display (OLED) or an organic light emitting diode.
The OLED is optimal for reduction in thickness of a light emitting device because it has a high visibility for its self-luminescence and thus it does not require a backlight which is necessary for a liquid crystal display (LCD). Moreover, the OLED is further advantageous in its unlimited viewing angle. For these advantages, light emitting devices using the OLED attract attention as display devices replacing CRTs or LCDs.
The OLED includes a layer containing an organic compound (organic light emitting material; such a layer is referred to as an organic light emitting layer, hereinafter), an anode layer and a cathode layer. The organic light emitting layer generates luminescence (electroluminescence) by applying an electric field across the anode and the cathode. The electroluminescence generated from the organic compound includes: light emission (fluorescence) caused upon return from a singlet excited state to a ground state; and light emission (phosphorescence) caused upon return from a triplet excited state to a ground state. The light emitting device of the present invention may use either one of the above-described types of light emission; alternatively, it may use both types of light emission.
In this specification, all layers provided between a cathode and an anode of an OLED are generically defined as organic light emitting layers. Specifically, a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer and the like are all included in the category of organic light emitting layers. The OLED basically has a structure in which an anode, a light emitting layer and a cathode are layered in this order. In addition to this structure, some OLEDs have a structure including an anode, a hole injection layer, a light emitting layer and a cathode layered in this order, and other OLEDs have a structure including an anode, a hole injection layer, a light emitting layer, an electron transporting layer, a cathode and the like layered in this order.
Such a light emitting device is expected to be used for various applications. In particular, the light emitting device is desired to be applied to portable equipment for its small thickness and thus the possibility of reduction in weight. For this propose, an attempt has been made to form an OLED on a flexible plastic film.
A light emitting device, in which an OLED is formed on a flexible substrate such as a plastic film, is advantageous not only in its small thickness and light weight but also its utility for a display having a curved surface, a show window and the like. Therefore, its application range is extremely wide, not limited to portable equipment.
However, a substrate made of plastic is generally likely to allow moisture or oxygen to pass through. Since the degradation of an organic light emitting layer is accelerated by moisture and oxygen, the lifetime of a light emitting device tends to be short by the penetration of moisture or oxygen. As a conventional solution of this problem, an insulating film such as a silicon nitride film or a silicon oxynitride film is provided between the plastic substrate and the OLED so as to prevent moisture or oxygen from penetrating into the organic light emitting layer.
In general, however, a substrate such as a plastic film is easily affected by heat. At an excessively elevated film formation temperature for an insulating film such as a silicon nitride film or a silicon oxynitride film, a substrate is likely to be deformed. On the contrary, at an excessively low film formation temperature, the quality of a film may be degraded, making it difficult to sufficiently prevent the penetration of moisture or oxygen.
Furthermore, if a thickness of an insulating film such as a silicon nitride film or a silicon oxynitride film is increased so as to prevent the penetration of moisture or oxygen, a stress is correspondingly increased to easily cause a crack in the film. Moreover, with an increase in thickness, a crack is likely to be generated in the insulating film when the substrate is bent.